1. Field of Invention
The present invention relates generally to hydraulic braking systems of motor vehicles and, more particularly, to fluid pressure regulating devices to be incorporated in such braking systems.
More particularly, this invention relates to a load-responsive pressure reducing valve assembly which is so constructed that a starting pressure for the pressure reducing operation thereof whereby an input hydraulic brake pressure from a master cylinder is changed to an output hydraulic brake pressure in a predetermined ratio, is controlled by means of a pressure proportioning valve which restricts the flow of fluid from the master cylinder to a brake wheel cylinder during a predetermined range of increasing fluid pressure supplied to the wheel brake cylinder. The device is capable of distributing the hydraulic brake pressure, corresponding to a transient weight transfer from rear wheels of the vehicle to the front wheels caused by the braking operation, to front and rear braking means by varying the starting pressure for brake fluid pressure reduction to the rear braking means in response to load and deceleration of the vehicle.
2. Discussion of the Prior Art
It is a well known fact that when a wheeled vehicle such as an automobile or truck having a brake system on the front and rear wheels is braked, the braking causes a forward transfer of the load which occurs due to the deceleration inertia of the vehicle and thereby reduces the load on the vehicle rear wheels and consequently reduces the contact of the rear wheels with the ground surface. Thus, the road holding ability of the rear wheels decreases, causing the rear wheels to lock under braking application which, in turn, causes dangerous skidding and possible loss of control of the vehicle. It is also known that this tendency is even more likely to be observed in small sized trucks which have a large ratio of load variation and which also have a short wheel base or distance between the front and rear wheel axles.
A number of attempts have been proposed for accomplishing the above-described pressure reduction operation, wherein the fluid pressure supplied to the rear wheel brakes is reduced to prevent such skidding and provide maximum brake efficiency under varying vehicle load conditions. One such apparatus is illustrated in U.S. Pat. No. 3,802,750, wherein the starting point of pressure reduction in a reducing valve is delayed under conditions of a heavy vehicle load by utilizing the characteristic that the vertical distance between the rear wheel axle and the loading platform of the vehicle is shortened with the application of the load. Another apparatus of the prior art is disclosed in U.S. Pat. No. 3,317,251, wherein the operation of the reducing valve is initiated only when the hydraulic pressure from the master cylinder is prevented from being transmitted to the rear wheel brake cylinders by means of an inertia ball valve member which moves in response to the deceleration of the vehicle.
Another known method is illustrated in U.S. Pat. No. 3,944,292, wherein the pressure reducing operation at the time of a heavy load condition is delayed by controlling the starting pressure of the pressure reduction operation of the reducing valve in response to or in accordance with the hydraulic pressure of the master cylinder at the time the inertia sensor senses the braking deceleration and moves. Another example of this type of sensor is also illustrated in U.S. Pat. No. 3,825,303. However, in load-responsive pressure reducing valves of this type, a shortcoming is observed when used in vehicles which display a tendency of not exhibiting any significant or definite difference in the relationship between braking hydraulic pressure and braking deceleration when it has a light load condition and when it has a heavy load condition. In this situation, the pressure reducing valve is thus apt to display the pressure reducing characteristic for a light load when, in fact, the vehicle is under a heavy load condition. This type of pressure reducing valve thus has the shortcoming that the braking force applied to the rear wheels tends to be insufficient under heavy load conditions notwithstanding its relatively sophisticated and complicated construction.
It is a principal object of the present invention to provide a load-responsive pressure reducing valve assembly which is devoid of the aforementioned disadvantages and to provide such a reducing valve assembly wherein the starting point of pressure reduction is controlled by detecting the deceleration of the vehicle and further aims at providing an inexpensive load-responsive reducing valve of simpler construction and which further allows variations in the starting point of pressure reduction in response to deceleration.